1. Field of the Invention
The present invention relates to solder alloys used for metal joining in electronic devices, more specifically to tin-antimony solder to solder down trimetal electrodes in electronic devices.
2. Description of the Related Art
Semiconductor devices such as diodes, thyristors, MOSgated devices such as MOSFETs, IGBTs and the like are commonly formed in silicon semiconductor dice containing the device junctions. The dice have metallized bottom drain or other power electrodes and have source and gate electrodes or other power electrodes on their upper surfaces. The dice are mounted on enlarged conductive lead frame pads or conductive segments of a printed circuit board or IMS; or DBC or the like by soldering and the power electrodes on the upper die surface have connection wires which are wire bonded by plural wires from the conductive electrode area of the dice to flat connection post areas which are in turn connected to the exterior lead conductors of the lead frame or other support.
In general, soldering is performed for the purpose of mechanical or electrical joining. The solder alloys are normally required to be superior in joining properties and corrosion resistance. In addition, the solder alloys desirably have a high thermal fatigue strength and a desired soldering temperature, and do not contain lead from the environmental point of view.
That is, lead in any form shows an internal accumulative toxicity. Therefore, problems of air pollution and waste treatment in the lead smelting process, accumulation in the physical bodies of babies and pregnant women due to exposure to the air and contamination of foods and the like are concerned.
It is also important that the solder alloys have a high thermal fatigue strength. This is because, since semiconductor device chips generate heat when powered, the solder joining a chip to a metal support is subject to a large thermal strain. The thermal strain is also generated in the soldering of the chips. Thus, the solder alloys forming the soldering part arc subjected to a rigorous operation environment.
Furthermore, the solder alloys are desirably those which are high in melting point and are not adversely affected by temperature excursions of subsequent processes.
Conventional solder alloys include tin-lead (Sn—Pb) alloy, tin-silver (Sn—Ag) alloy and tin-antimony (Sn—Sb) alloy.
Since the tin-lead (Sn—Pb) alloy is low in tensile strength and superior in ductility, it is high in strain generation and low in fatigue strength. Therefore, in conjunction with its low heat resistance, it is low in thermal fatigue strength. The Sn—Pb alloy has an eutectic temperature of 183° C. The melting point can be increased from 183° C. to the vicinity of 300° C. by increasing the Pb content. However, since this widens the solid-liquid coexistence area between liquid phase temperature and solid phase temperature (183° C.) and the eutectic temperature is 183° C., it has problems in that it is low in heat resistance and tends to undergo material degradation at relatively low temperatures. The external solder is Sn—Pb. The internal solder alloy of the package is Sn—Ag, e.g. Sn 96.5%-Ag 3.5%. The liquid phase solid phase temperature is 221° C. In addition, when Sn—Ag alloys are used to solder trimetal surfaces such as Ti—Ni—Ag, the titanium layer gets attacked (i.e. oxidized) when the nickel and silver layers are dissolved.
The Sn—Sb alloy is relatively higher in strength and is thus better than the Sn—Ag alloy. The Sn—Sb alloy contains 8.5% by weight of Sb, has a peritectic point at 245° C., and Sb is used normally in an amount of less than 8% by weight. Since melting takes place between melting temperature 232° C. of Sb and the peritectic temperature 245° C., the solid-liquid coexistence area is small, the heat resistance is good, and one which is high in strength can be obtained by increasing the Sb content. However, the Sn—Sb alloy has problems in that it becomes degraded in processability particularly when the Sb content is increased, and becomes low in wettability at soldering. Furthermore, the prior art teaches (see U.S. Pat. No. 6,179,935) that Sn—Sb alloys show considerable formation of oxide film which is a particular problem in soldering trimetal surfaces. Accordingly, no Sn—Sb alloy was ever used to solder down trimetal surfaces. Instead, a more conventional Sn—Pb was used for these surfaces despite the presence of the above-identified disadvantages including also the formation of oxide films.